The invention relates to acoustic transducers, and more particularly, to a robust, high power, ultra-low frequency acoustic transducer design.
Acoustical transducers convert electrical energy to acoustical energy, and vice-versa, and can be employed in a number of applications. In the detection of mobile vessels, for example, acoustic transducers are the primary component of sonar devices, and are generally referred to as projectors and receivers. Projectors convert electrical energy into mechanical vibrations that imparts sonic energy into the water. Receivers are used to intercept reflected sonic energy and convert the mechanical vibrations into electrical signals. Multiple projectors and receivers can be employed to form arrays for detecting underwater objects.
In a typical application, marine seismic vessels tow vibrators and discharge air guns, explosives and other acoustic projectors to generate seismic energy in marine geophysical testing. The seismic energy comprises a pressure pulse that travels through the water and underlying subsurface geologic structures. The energy is partially reflected from interfaces between the geologic structures and is detected with geophone or hydrophone sensors.
Conventional transducers, however, are associated with a number of unsolved problems. For instance, currently known transducer designs are generally not capable of producing large amounts of acoustical energy at low frequencies on the order of two kilocycles or less, and in particular, under 400 Hz. Similarly, there appears to be no transducer that operates with considerable efficiency so as to provide large power outputs over low frequency ranges.
Physical limitations on transducer design further complicate solving such deficiencies. For example, effective mechanical stress management is important for deep depth capability, as well as for the ability to produce high acoustic power levels.
Generally, the family of sonar projectors capable of generating low frequency operate in a wall flexure mode. These projectors include flextensionals, inverse flextensionals, bender discs, wall-driven ovals (also know as xe2x80x9cWALDOsxe2x80x9d), and slotted cylinder projectors. Slotted cylinders can typically operate at frequencies lower than the frequencies at which flextensionals and WALDOs can operate given a fixed wall thickness and effective diameter.
However, the achievabie low frequency range of such slotted cylinders is still limited (nothing below 400 Hz), given the current need for low frequency transducers. Lower frequencies can be obtained by thinning the transducer wall thickness. On the other hand, as the wall thickness is decreased, mechanical stresses due to the wall flexure increase. For flextensionals and WALDOs to match the lower frequency capability of slotted cylinders, their walls would have to be thinned to a point where hydrostatic pressures would compromise their structural integrity.
What is needed, therefore, are robust, ultra-low frequency acoustic transducer designs.
One embodiment of the present invention provides an acoustic transducer configured for producing ultra-low frequency, high power coherent acoustic radiation. The transducer includes a projector shell having an oval cross-section with a short axis and a long axis, and a slot opening on the short axis. The outer diameter of the projector shell is at least 18 inches along the short axis. A plurality of active transducer elements are disposed along the internal surface of the projector shell, with the transducer elements adapted for coupling to a power source. The transducer operates in the frequency range under 400 Hz.
The transducer may further include an internal cylinder having an outer diameter that is less than an inner diameter of the projector shell. End caps coupled to each end of the internal cylinder secure the projector shell in place about the internal cylinder. The active transducer elements can be retained by a groove on the internal surface of projector shell. The transducer may also include a flexible water-proof material covering the projector shell or shells that is adapted to keep the active transducer elements dry in conjunction with the end caps.
Alternative embodiments may include a plurality of projector shells that are coupled to one another with their respective slot openings aligned, with each projector shell having a plurality of active transducer elements disposed along its internal surface. The plurality of active transducer elements may include, for example, at least one of piezoelectric elements, ferroelectric elements, and rare earth elements. The projector shell can be, for instance, at least one of a solid metal, solid composite, honey comb metallic, and honey comb composite.
In one particular embodiment, each projector shell has a thickness (e.g., 6 inches or less) that allows the transducer to operate in a frequency range below 120 hertz. The projector shells can be operatively coupled to form an array of acoustic projector modules that produces coherent high powered acoustic radiation. Generally, the acoustical power provided by the array can be at least doubled by doubling the number of projector shells included in the array.
Another embodiment of the present invention provides a method of manufacturing an acoustic transducer that is configured to produce ultra-low frequency, high power coherent acoustic radiation. The method includes providing a projector shell having an oval cross-section with a short axis and a long axis, a slot opening on the short axis, and an outer diameter of at least 18 inches along the short axis. The method further includes disposing a plurality of active transducer elements along the internal surface of the projector shell. The transducer elements are adapted for coupling to a power source. The transducer operates in the frequency range under 400 Hz.
The method may further include providing an internal cylinder having an outer diameter that is less than an inner diameter of the projector shell, and connecting an end cap to each end of the internal cylinder so as to secure the projector shell in place about the internal cylinder.
In alternative embodiments, providing a projector may include providing a plurality of projector shells that are coupled to one another with their respective slot openings are aligned, each projector shell having a plurality of active transducer elements disposed along its internal surface. Such embodiments may further include providing an internal cylinder having an outer diameter that is less than an inner diameter of the projector shells, and connecting an end cap to each end of the internal cylinder so as to secure the projector shells in place about the internal cylinder.
In one particular embodiment, providing a projector shell includes providing one or more projector shells each having a thickness that allows the acoustic transducer to operate in a frequency range below 120 hertz. The method may further include covering the projector shell or shells with a flexible water-proof material or boot that is adapted to keep the active transducer elements dry. In another particular embodiment, disposing the plurality of active transducer elements includes disposing the active transducer elements in a groove on the internal surface of the projector shell.
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.